Extracting the Symmetry Energy Information with Heavy Ion Collisions

Key words: 
• symmetry energy /
• heavy ion collision /
• collective flow /
• stopping power

Abstract: Nuclear symmetry energy plays an important role in the properties of nuclei and neutron stars. Since theoretical predictions of the density dependence of symmetry energy S(ρ) from microscopic nucleonnucleon interactions show large uncertainties, especially in the region of suprasaturation density, constraining the density dependence of symmetry energy has become one of the main goals in nuclear physics and has stimulated many theoretical and experimental studies. In this paper, we have reviewed the ImQMD05 code and its description on charge distribution, collective flow and nuclearnuclear stopping. By comparing ImQMD05 prediction with data, the isoscalar part of nucleonic mean field, the energy and density dependence of inmedium NN cross sectoin in the ImQMD05 were determined. In order to extracting the symmetry energy information by comparing the ImQMD05 calculations with the data for 112, 124Sn+112, 124 Sn at Ebeam=50 MeV/u, the influence of symmetry potential and inmedium NN cross section on the isospin sensitive observables of intermediateenergy heavyion collisions was investigated. Focusing on the region above the Fermi energy, our results show that the symmetry potential plays a more important role in the experimental observables, such as double neutron to proton ratio and the isospin transport ratio Ri, than that the inmedium nucleonnucleon cross section does. Since the copious production of intermediate mass fragments is a distinguishing feature of intermediateenergy heavyion collisions, we also examined the influence of cluster emission on the isospin transport ratio using different isospin tracers. The values of the isospin transport ratios with the tracer defined by the isospin asymmetry of the heaviest fragments with Z≥20 in the projectile region is greater than those obtained from projectile residues (emitting source). This phenomenon can be tested experimentally. By comparing the ImQMD05 predictions with the data for three observables, the selfconsistent constraints on the symmetry energy at subsaturation density were obtained.